[alfa] head porting, flow reversal

[AlfaNeely@xxxxxxx]

In a message dated 10/4/2004 7:24:33 PM Central Daylight Time, 
owner-alfa-digest@domain.elided writes:
Date: Mon, 4 Oct 2004 13:15:26 -0400
From: Joe Elliott <jee@domain.elided>
Subject: [alfa] head porting, flow reversal


>  It's a given that to leave a step on the exhaust port can help 
>reduce flow reversal with cams that have a large amount of overlap. 
>Surely then the same would apply to the inlet port? Also the sudden 
>pressure drop at that point may induce more flow than a gradual 
>taper would.

Is what they say about the exhaust port/header interface true?  If 
so, which way are they talking about stepping it--bigger header than 
port, or vice versa?  And why does this reduce flow reversal (which I 
assume happens because the exhaust manifold is at a higher pressure 
than the intake manifold).

Thanks,
Joe Elliott
'82 GTV-6 
-------------------------------

    The Anti Reversion headers sold by Jack Beck www.orionmotorsports.com 
have a short pipe that exits the exhaust port about an inch.  Surrounding that is 
a huge pipe that tapers back down to the nominal diameter of the exhaust 
port.  That creates a chamber that looks rather like the megaphone exhaust pipes 
you see on some motorcycles.
    When the sound wave created by the exhaust reaches the end of the pipe, 
it "echoes" back down the length of the pipe.  If the reversion wave reaches 
the exhaust valve at exactly the wrong time, it can push exhaust back into the 
cylinder.  It creates a high pressure at the wrong time.
    A "tuned" exhaust can and will help pull exhaust out of the cylinder.  
That is when the reversionary pressure wave returns to the exhaust valve at 
exactly the correct instant to create a low pressure that will help draw the 
exhaust out of the cylinder.
    The problem is, a tuned exhaust only works at a certain RPM and multiples 
of that RPM.  An anti reversion exhaust creates a huge chamber where the 
exhaust reversion is killed before it can reach the valve.  That kills the tuned 
exhaust effect, but makes the exhaust work across a wide RPM range.

    The street headers sold by several of the catalog vendors are crap.  The 
front of the exhaust pipe shrouds the exhaust port on all four cylinders.  If 
you can easily get a standard exhaust nut to fit the headers, then the exhaust 
is shrouded.  
    These pipes all have a step built into their design.  The problem is the 
step goes the wrong direction.  I know the headers from IAP and Centerline had 
that problem.
    The headers from Alfa Ricambi used to be great, but I am not sure they 
are in business this week.  I will probably buy from Paul Spruell or Jack Beck 
next set I buy.


Ciao,
Russ Neely
Oklahoma City
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